Fuel injection systems deliver fuel to the combustion chamber of an engine, where the fuel is thoroughly mixed with air before combustion. One form of fuel injection system well-known in the art is a fuel spray nozzle. Fuel spray nozzles atomise the fuel to ensure its rapid evaporation and burning when mixed with air.
An airblast atomiser nozzle is a type of fuel spray nozzle in which fuel delivered to the combustion chamber by a fuel injector is aerated by swirlers to ensure rapid mixing of fuel and air, and to create a finely atomised fuel spray. The swirlers impart a swirling motion to air entering the combustion chamber, so as to create a high level of shear in the fuel flow.
Typically, an airblast atomiser nozzle will have a number of swirlers. An annular fuel passage between a pair of swirlers feeds fuel onto a prefilming lip. Thus a sheet of fuel is formed that breaks down into ligaments. These ligaments are then broken up into droplets within the shear layers of the surrounding highly swirling air, to form the fuel spray stream that is emitted from the fuel injection system.
Combustion noise in a gas turbine combustor is encountered when the fluctuating heat release occurring within the combustor is in phase with the resonant frequency of the combustor cavity. The resulting fluctuating pressure can lead to unacceptably high levels of audible noise and deterioration of component life through excessive cyclic loading. It is thought that poor aerodynamic flow within the fuel injector (resulting in local flow instabilities and recirculations within the injector) is linked to the occurrence of combustion noise.
Typically, two frequency ranges of combustion noise are encountered: one is termed Low Frequency Rumble (LFR) and occurs at frequencies in the range of 70-170 Hz, whilst the second form is called High Frequency Rumble (HFR) and typically occurs in the frequency range of 400-600 Hz. Generally, combustion noise is encountered during some transient engine manoeuvres or over a limited range of engine steady state operation. Therefore, an option to combat combustion noise is to vary the fuel supply schedule so that regions within the engine's operating envelope where combustion noise is encountered are either not entered, or, as combustion noise is known to be closely linked to the richness of the air fuel ratio within the combustor, are traversed with reduced fuel flow. For example, decreasing the air-fuel ratio of a fuel injector can be an effective way of preventing particularly LFR. Alternatively, devices such as Helmholtz resonators and passive dampers can be attached to the combustor cavity to damp the amplitude of the fluctuating pressure component.
However, aircraft operability requirements generally dictate the operation mode of the engine. Thus altering the fuel schedule such that the engine operates away from or at reduced fuel flow at a point associated with combustion noise is seldom a viable option. The attachment of resonator or damping devices to a combustor can be effective in attenuating combustion noise. However, such devices have to be durable enough to survive the combustor operating environment, and their inclusion adds weight and cost to the engine. Further, the inclusion of such devices can be hindered by space restrictions on the combustor wall due to the presence of air admission ports. Also, resonators are narrow band devices and their effectiveness can be compromised if the combustion noise does not coincide with an anticipated frequency range. Although enriching the fuel injector has been shown to be effective for combating LFR, it has proved less effective in combating HFR and can result in high levels of smoke. Accordingly, there is a continuing need to develop fuel injection systems to combat combustion noise.